Regenerative afterburner for air pollution elimination

ABSTRACT

An afterburner including a regenerator disk having passages therethrough disposed over the combustion chamber, means slowly rotating the disk, a inlet duct having a first rigid seal extending completely about the periphery of the upper surface of the disk, and an exhaust duct extending through the inlet duct, the duct chamber having a second seal extending about a sector of the upper surface of the disk within the first seal.

United States Patent 1 1 Foster-Pegg Feb. 27, 1973 [541 REGENERATIVE AFTERBURNER FOR AIR POLLUTION ELIMINATION [76] Inventor: Richard W. Foster-Pegg, 101 Fourth Street, Warren, Pa. 16365 [22] Filed: Jan. 27, 1971 [21] Appl. No.: 110,279

Related US. Application Data [63] Continuation-impart of Ser. No. 838,544, July 2,

1969, abandoned. 1

301 Foreign Applicatihn i rioritybata July 5, 1968 Great Britain ..32225/68 52 us. 01...... ..23/277 c, 23/284, 423 245, 165/57, 165/9, 110/8 A, 55/D1G. 30 51 1111. c1. ..F23 7/06, F231 5/02 [58'] Field of Search 23/ 277 C, 2 C, 284; 165/5, 165/7, 9; 110/8 A; 55/DIG. 30

[56] References Cited UNITED STATES PATENTS 3,509,834 5/1970 Rosenberg et al. ..1 10/8 A Primary Examiner-James H. Tayman, Jr. AtlomeyWilliam A. Drucker [57] ABSTRACT An afterburner including a regenerator disk having passages therethrough disposed over the combustion chamber, means slowly rotating the disk, a inlet duct having a first rigid seal extending completely about the periphery of the upper surface of the disk, and an exhaust duct extending through the inlet duct, the duct chamber having a second seal extending about a sector of the upper surface of the disk within the first seal.

2 Claims,- 8 Drawing Figures PATENTED 3.718.440

SHEET 2 [IF 3 PATENTED FEB 2 7 I975 SHEET 30F 3 REGENERATIVE AFTERBURNER FOR AIR POLLUTION ELIMINATION CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my now abandoned application Ser. No. 838,544 filed July 2, 1969, entitled REGENERATIVE AFTERBURNER FOR AIR POLLUTION ELIMINATION.

BACKGROUND OF THE INVENTION Incineration of gaseous contaminants can serve to accomplish many of the desirable objectives in air pollution control and produce other beneficial results. For example, odor problems created by-rendering plants, kraft pulp mills, baking oven operations, and by the release of mercaptans from different industrial processes can be eliminated with the proper application of flame incineration equipment. The opacity of the gaseous products emitted from coffee-roasting operations, paint-baking ovens, smokehouses and diesel engines can be reduced. Hydrocarbon emissions from numerous operations such as paint-spray booths, storage tank vents, and internal combustion engine exhausts can be burned to carbon dioxide and water vapor with afterburning equipment. For complete corn bustion of organic vapors and gases or flamable liquids and even solids to take place, it is necessary that materials to be burned react with oxygen at a sufficient temperature and for a sufficient length of time for the reaction to be completed.

Combustible gases that are to be completely burned must be elevated to a temperature above their minimum ignition temperature in the presence of oxygen. Minimum ignition temperatures vary from 580F for acetylene to 1,170F for methane. Under ordinary circumstances, however, atemperature of l,500F in the presence of excess air will destroy almost any organic material or aerosol. The general range of temperatures utilized successfully in afterburner devices is 1,200 to 1,500F.

Oxygen is required for complete combustion. Typically, when incomplete combustion occurs, an air pollution problem is not solved; it is aggravated. Complete combustion is achieved when sufficient air is supplied to enable the theoretical quantity of oxygen to intermix with the vapor to be burned. Excess air is provided to make certain that more than the stochiometric quantity of oxygen is provided. Also, the time factor, referred to as residence time or retention time in the combustion device, is most important if complete combustion is to take place.

Of particular significance is satisfactory control of the contaminant flow rate. It is undesirable to feed an afterburner with a waste gas-oxygen mix that is in the explosive range for the particular gases involved inasmuch as an explosion can result in the system. Accordingly, the contaminated gas stream must be maintained at a concentration level-below the lower explosive limit.

BRIEF'DESCRIPTION OF THE DRAWING FIG. 1 is a side view of a single disk afterburner unit according to this invention; 1

FIG. 2 is a longitudinal vertical section through a fragment of the upper portion of the unit of FIG. 1;

FIG. 3 is a top view of a multi-disk afterburner;

FIG. 4 is a longitudinal vertical section taken on line 4-4 of FIG. 3;

FIG. 5 is a longitudinal vertical section through a fragment of the disk shaft and the bearing assembly therefor;

FIG. 6 is a top view of a regenerating disk showing areas through which gases may pass in the device of this invention;

FIG; 7 is' a top view of a fragment of a regenerative disk; and

FIG. 8 is a top view of a fragment of a wall of an inlet chamber showing a manifold extending therethrough over a fragment of a-disk.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIGS. 1 and 2, the regenerative afterburner of this invention has a compressor or fan 10 driven by a 50 h.p. motor 11 by the belt and pulley drive 12. Duct 13 of fan 10 receives contaminated gases at 500F at ambient pressure and compresses them to about 27 inches of water to pass them through ducts l4 and 15 which lead to the combustion chamber 16. Chamber 16 has a metal shell 17 lined with refractory material 18 and supported by the legs 19. Inlet duct 20 is connected to duct 15 and disposed over the entire top of combustion chamber 16. Within a circular opening at the top of chamber 16 there is disposed a regenerative disk or wheel 22 mounted on a shaft 23.

As may be seen in FIG. 5, shaft 23 is connected to an extension of the hollow shaft 24 which forms a labyrinth seal 25 with housing 26. Hollow shaft 24 is journalled in a roller bearing 27 which rotatably mounts disk 22. As is further shown in FIG. 2, shaft 24 is driven by a motor 28 through the reducing gear box 29. A rotary union 30 on top of hollow shaft 24 is connected by means of a line 31 to any convenient source of cool compressed air at 20 to 30 p.s.i. Housing 32 supports gear reducer 29 and motor 28.

' An exhaust duct 35 is disposed over disk 22 opposite the largest portion of andwithin inlet duct 20. A metal inlet chamber seal 36 extends completely about the periphery of disk 22. A partition 39 divides the inlet duct 20 from the exhaust duct 35. A portion 40 of partition 39 surrounds shaft 23 and opens into inlet duct 20 so that shaft 23 will operate in the cooler inlet gases.

Cool air entering line 31 passes through hollow shaft 24 and its lower extension to flow through the apertures 41 in the labyrinth seal 25. This air cools bearing 27 as it leaks from seal 25 and it prevents any upward flow of hot gases through seal 25. An adjusting nut 42 secures disk 22 to shaft 23 and a radiation shield 43 reduces heat flow up shaft 23. An exhaust duct 44 leads to a stack 45.

As shown in FIG. 7, disk 22 is a refractory heat transfer material able to withstand temperatures in excess of 2,000F. Disk 22 contains a number of through passages 46 which provide an open area of over 70 percent. One example of such material is Cercor manufactured by Corning Glass Works. One such disk was 27 This invention operates as follows. Satisfactory combustion of pollutants in an afterburner requires the pollutants be raised to a temperature from l,000 to 1,500F. By use of fuel alone, 20 Btu are required to raise the temperature of 1SCF l,OF. The cost to eliminate pollutants from 5,000 SCFM of 500F stack gas by combustion to l,500F for 8,000 hours per year with gas at 30 c per MCF, would be I 7,000 per year.

The purpose of this invention is to reduce the requirement for fuel of an afterburner system by recycle of exhaust heat to preheat the incoming gas.

The performance of regenerators is best compared on the basis of effectiveness defined as the temperature rise in or heat transferred by the regenerator compared to the theoretical maximum which could be transferred in an exchanger with infinite area. For example, with a gas inlet of 500F and exit from the afterburner of 1,500F, an exchanger with infinite surface and 100 percent effectiveness could preheat the inlet gas to l,500F for a temperature rise of 1,000F. A regenerator with 75 percent effectiveness would prehead the inlet gases 75 percent of 1,000F or 750F, thus reducing the fuel requirements from 1,000F temperature rise to 250F temperature rise and saving 75 percent of the $17,000 annual fuel bill required without regeneration. The inlet gases would be preheated to l,250F and temperatures of disk 22 would reach an average between L250 and l,500 or about 1,400F with higher temperatures during upsets and transients.

Best performance of a regenerator is obtained when heat is stored in the wheel without conduction across or around the wheel, thus the low conductivity of the disk 22 material is desirable.

Seals 36 and 37 are required to minimize leakage of gas around the wheel 22 to and from the combustion chamber 16 and also across the partition 39 from inlet duct to the exhaust duct 35. Non-rubbing, close clearance seals 36 and 37 allow minimal leakage.

To maintain the intended seal clearance, the wheel shaft 24 and the structural members connecting the seal 36 and the wheel bearing 27 are all surrounded by the cooler inlet gas, thus these elements are maintained at equal temperature and expand equally. The hotter exhaust gas is contained in the exhaust duct 35 which is surrounded and separated from the main structure by the cooler inlet gas. Pressure in the combustion chamber 16 is less than in the inlet duct 20, thus, leakage across the peripheral seal 36 is of cooler inlet gas leaking into the combustion chamber 16. In this way hot gas is kept away from the seals 36 and 37 which are prevented from overheating.

To provide for close control and ease of adjustment of clearances, the disk 22, seals 36 and 37, shaft 24, bearing 27, and motor 28 are a precision assembly and are inserted and withdrawn into the complete regenerator as a single unit. Wheel or disk 22 sub-assemblies will be interchangeable between units of different numbers of wheels 22.

To prevent overheating of the wheel shaft bearing 27 by conduction of heat along the shaft 24, the shaft 24 is cooled by cool pressurized air from line 31. The cooling air leaks out in the labyrinth shaft seal 25, with a portion of it leaking inwards into the gas inlet duct 20 so that leakage of polluted gas to the outside is prevented.

The flow reversal, which occurs in the wheel 22 once per revolution, tends to prevent deposit and fouling. Combustible deposits can be burned off the wheel 22 by intentional overheating of the wheel 22 by slowing or stopping its rotation.

Afterbuming requires oxygen to be present in the gases at time of combustion. Many polluting gases do not contain oxygen and require addition of air for combustion proportional to the fuel required to be burned. The regenerated afterburner of this invention requires the addition of less air than a straight afterburner with a greater fuel saving than for cases where no air addition is required.

In this rotary regenerator the direction of gas flow reverses as the wheel 22 passes over the partition 40 between inlet and exhaust ducts 20 and 35. At time of reversal, the gas contained in the wheel passages 46 is blown back. Gas on its way out of the combustion chamber 16 is blown back into the chamber 16 and the inlet gas flowing into the chamber 16 is blown back into the exhaust without passing through the chamber 16. As shown in FIG. 6, area 48 represents the area of disk 22 through which gases flow from inlet duct 20 and area 49 represents the area of disk 22 through which gases flow into exhaust duct 35 from disk 22. If addi-v tional air is required, it may be introduced through a separate sector 50' of wheel 22 by manifold 201 connected to air supply 200 shown in FIG. 8. Sector 50 follows the inlet sector 48 so that air introduced through this sector 50" purges the disk 22 of inlet gases so that blow out loss of untreated gases from disk 22 is prevented.

The afterburner is supplied with a fan 10 to balance its pressure drop. Pressure drop during both passes through the disk 22 is about 22 inches of water which requires a fan 10 of about 50 h.p.

Typical operating conditions for a unit as shown in FIG. 1 are as follows. Polluted gas flows to the fan inlet duct 13 at 500F and approximately at atmospheric pressure. The fan 10 raises the pressure of the gas to 27 inches water required to push the gas through the regenerator disk 22. In passing through the wheel 22, the gas is heated from 500 to l,300F, thus cooling the wheel 22. Combustion in chamber 16 then raises the temperature from l,300 to l,500F. Combustion chamber volume will provide approximately 0.5 secs. residence time at maximum temperature. When leaving the chamber 16 through the regenerator wheel 22, the gas cools from l,500 to 700F while heating the wheel 22. I

The wheel 22 is rotated at about 15 RPM by speed reducer 29 driven by A HP motor 28. To permit the wheel 22 to be slowed for cleaning, a variable speed motor 28 is supplied. As shown, the gas discharges direct to stack 45, but can alternately be passed through a heat exchanger (not shown) to heat water or raise steam for heating and air conditioning.

The unit described will treat about 5,500 SCFM (25,000 lb/hr. or 7 lb/sec) of polluted gas.

The fan motor 11 absorbs 50 HP, and the motor 28 to turn the regenerator wheel 22 takes about A HP. Fuel consumption is about 1,600 SCF of natural gas per hour introduced through line 50, insulating bushing 51 and burner 52 as shown in FIG. 2. This unit with fan and motor and all accessories mounted on a skid weighs about 15,000 lbs. The skid is about 12 ft. X 6 ft. Total height of the unit is about 16 ft.

With good combustion conditions, no fouling of the regenerator disk 22 has occurred. When combustion of liquid fuels has been inefficient because of maladjustment of the combustion equipment, the disk 22 has fouled. If a pure hydrocarbon distillate is being burned,

the deposits are all combustible and predominantly carbon and can be burned from the regenerator disk surface by raising the temperature to above the ignition point. In normal operation in the afterburner, the hot side of the wheel 22 will operate above the ignition temperature of deposits and no deposition will occur on the hot side. The cold side will operate at a mean temperature between gas inlet and outlet temperatures which will usually be below the ignition temperature of deposits, thus deposits will form in a zone extending part way through the wheel 22 from the cold side. The temperature of the cold side of the wheel 22 can be raised to burn off the deposits by slowing the wheel to spoil the effectiveness of heat exchange and simultaneously increasing the afterburner firing. Deposits can also be removed by stopping the wheel 22 in a series of positions until all sectors had been cleansed, however, the unequal heating of the wheel 22 may result in detrimental thermal stresses and distortion.

The combustion temperature rise in the regenerative afterburner if a fraction of the total temperature increase of the gas equal to l-E percent. Where E is the regenerator effectiveness,'E will usually fall between 70 and 80 percent, thus, total temperature rise will be from three to five times the combustion temperature rise. Temperatures in the afterburner will usually not exceed 2,000F, thus combustion temperature rise will fall between 200 and 500F. This temperature rise requires a heat release by combustion of from 5 to l l Btu per SCF. If the effluent gas to be treated contains this quantity of unburned combustible after the regenerator has achieved operating temperatures, no supplementary fuel will be required. Preheating-and pilot fuel will be required depending on operating conditions.

The lower explosive limit of a gas (LEL) will usually contain combustibles that will result in a temperature rise of about 50 Btu per SCF.

To provide a margin of safety, combustible contents are held to a maximum of 25 percent of the LEL or a maximum heating value of about 13 Btu per SCF. Maximum temperatures in the regenerative afterburner will be exceeded if combustibles in the gas exceed about Btu per SCF, thus explosions and flashback in the regenerative afterburner are unlikely to be a factor with sound application and operating engineering.

.The quantity of fuel required by the regenerative afterburner when evenly dispersed throughout the gas stream is not combustible at normal temperatures. At the higher temperatures after passing through the wheel 22, the dispersed fuel often will burn, and it is often desirable to inject the fuel gas through injector 60 shown in phantom lines in FIG. 2, upstream of the wheel 22 where it is not combustible and thus use the wheel 22 as a flame holder. This makes the best use of combustor volume and results in a low cost system. A flame holder and ignitor (not shown) are then required downstream of the wheel for light off and warm up.

FIGS. 3 and 4 show a modification of this invention in which a large volume combustion chamber lined with refractory material 101 has a large inlet duct 102 fixed over it and connected to a fan (not shown). Four identical units 103 are fixed about the periphery of chamber 100 in duct 102. Each unit 103 has a drive 104 substantially identical to those described for the first embodiment of this invention. Each drive 104 rotates a regenerator disk 105. Four exhaust ducts 107 extend over a sector of each disk 105 and communicate with the central stack 108. The exhaust ducts 107 thus extend through the inlet 102. This modification of the invention operates in the manner described for the first embodiment except that its capacity is increased four times. The units 103 may be independently replaced for servicing.

While this invention has been shown and described in the best forms known, these are purely exemplary and modifications may be made without departing from the spirit of the invention.

What is claimed is:

l. A regenerative afterburner of gases for air pollution elimination comprising, in combination, a combustion chamber, a regenerator disk of low heat conductivity having passages extending therethrough disposed over said combustion chamber, an inlet duct having a first seal extending about the periphery of said disk, means for slowly rotating said disk, an exhaust duct having a second seal extending about a first sector of said disk within said first seal, said exhaust duct extending to said first sector of said disk, a fan forcing gases from said inlet duct through said disk into said combustion chamber heating said gases, said gases burning in said combustion chamber and flowing through said disk into said exhaust duct heating said disk and means in said inlet duct over said disc having an air supply for purging said disk of inlet gases prior to said disk rotating under said second seal of said exhaust duct.

2. The combination according to claim 1 wherein there is a plurality of said disks and means rotating said disks mounted about the periphery of said combustion chamber, said inlet duct being a single duct extending over all said disks, said exhaust duct being a plurality of exhaust ducts extending within said inlet duct, each exhaust duct being disposed over a centrally disposed first sector of each of said disks, and with the addition of a central stack, said exhaust ducts communicating with said central stack, said means for purging extending into said single inlet duct for each of said disks. 

2. The combination according to claim 1 wherein there is a plurality of said disks and means rotating said disks mounted abouT the periphery of said combustion chamber, said inlet duct being a single duct extending over all said disks, said exhaust duct being a plurality of exhaust ducts extending within said inlet duct, each exhaust duct being disposed over a centrally disposed first sector of each of said disks, and with the addition of a central stack, said exhaust ducts communicating with said central stack, said means for purging extending into said single inlet duct for each of said disks. 